1. Field of the Invention
The invention relates to a process and a device for plating, continuously and at high speed, a long span of metal such as wire, tubing, round or flat metal strips with a layer of metal. More particularly, it applies to nickel plating of aluminum wire used in electrical applications.
2. Description of the Prior Art
There exist numerous conventional processes for plating metal parts with other metals in order to improve the surface properties of the metal parts such as appearance, resistance to corrosion, and contact electrical resistance. These processes are based on a variety of techniques such as deposition by metal plating, by plasma, in the vapor phase, by chemical means, by coating, by co-lamination or co-extrusion, or by electrolytic means.
Depending on the type of metal to be plated, its surface characteristics, the type of plating, the types of constraints imposed by the device for implementing the process, and the characteristics desired of the final product, all of these processes display advantages and disadvantages. The process representing the best compromise must be selected as a function of the final objectives.
In the present case, the principal objective is to provide a solution to the problem of plating aluminum or aluminum alloys with electric conductors. It has been known for a number of years that aluminum and its alloys, and, more particularly, the alloy designated as 6101 by the Aluminum Association, is similar to copper both in terms of electric resistivity and mechanical characteristics. However, its use in the form of nickle is not recommended in connection systems currently used in electrical equipment, and more particularly in high-use or high-temperature applications. Under these conditions, an increase of contact resistance is observed, which may lead to overheating, which is detrimental to the durability of this type of conductor and to equipment safety. In order to benefit completely from the indisputable advantages of aluminum and to promote its universal use instead of copper in conductors, it was therefore necessary to design an economical process for providing a stable contact resistance for the wire which would be at least equal to that of copper in the long term.
Other attempts have been made to promote increased use of aluminum conductors and to dispel the prejudices of manufacturers of electrical equipment who are reluctant to use aluminum. Other manufacturers and aluminum users have attempted to develop technological improvements designed to solve this contact resistance problem. The following solutions have been proposed:
co-lamination and co-extrusion processes, the development of which has been limited due to high manufacturing costs;
electrolytic tin coating processes, which have not become widespread due to time-consuming preliminary metal preparation requiring undercoating with bronze and/or copper by immersion in cyanide baths and to the rising price of tin, which has become a strategic metal.
Therefore, a trend has emerged in recent years favoring the use of plating with nickel--a metal which is far less expensive than tin, yet which intrinsically possesses good resistance to high temperatures, while continuing to plate with electrolysis, a practice well-adapted to aluminum.
Consequently, there has emerged in this field a number of processes involving cells in which the electrolyte circulates at high speed, or which use a more or less complex surface preparation, or intermediary layers for galvanizing. All of these methods produce a relatively adhesive coating, but display one major disadvantage: relatively slow processing speeds, usually limited to a few meters per second. Moreover, these methods require the use of extremely long devices, the production of which involves significant investment costs, to obtain sufficient immersion time in the electrolytic solution.
French Pat. No. 2,012,592 teaches that it is possible to coat aluminum wire with a 3 .mu.m layer of copper, at a line speed of 30 meters per minute, first by drawing it thorough a peripheral drawplate, then through an electrolytic bath of 3 m. in length to which an electromotive force is applied through an anode contained in this electrolytic solution with the wire acting as a pure cathode. Although the time of immersion in the electrolytic solution is only six seconds for a thickness of 3 .mu.m, the coating is formed of copper. Therefore, the results of the present process using nickel could not be predicted, particularly with regard to adhesion and contact resistance.
In an attempt to apply this teaching to nickel plating of aluminum, without attention to the method of planing, in a 5 m long bath; difficulties were encountered, particularly in terms of the supply of electric power to the wire. All of the devices employed--wheels, rollers, friction contacts--produced electric arcs of increasing magnitude that became increasingly detrimental to the adhesion of the plating with higher line speeds, requiring a reduction of current density, and, consequently, reduction of speed to obtain a layer of plating of sufficient thickness. Indeed, the maximum speeds attainable were approximately 25 m/minute, or an immersion time of 12 seconds for a thickness of 0.5 .mu.m, for a nickel-plated wire which did not entirely conform to established standards.
Therefore, a need continues to exist for a process by which long spans of metal, particularly those made of aluminum or aluminum alloys, may be coated with an adhesive metal layer, at a high speed with a relatively short immersion time in the electrolyte, to produce a coated metal span having a thickness and contact resistance which conforms to the various standards established in the electrical industry.